I'm not going to shed too much light on the history of seatstays, maybe somebody from the framebuilder's forum can nail this down (I'd like to know, too). But, in a nutshell there were very thin diameter "pencil stays" popular in early years which gave way to more fashionable thin but double tapered (top and bottom) stays that were either 12mm diameter if Columbus or 1/2" if Reynolds. At some point beefier (16mm or 5/8", respectively) stays came and went in the fashion parade and in the case of Reynolds they were single taper (maybe Columbus, too, but I dunno).
Since my knowledge of all this is pretty sketchy I look forward to being corrected
